1. Field of the Invention
The present invention relates to a road map displaying system for an automotive vehicle, which can display various road information received through communication on a road map related to the route along which the vehicle is now travelling, and more specifically to an improvement in reliability of road information displayed on the road map.
2. Description of the Prior Art
There has been known a road map displaying system for an automotive vehicle for guiding the vehicle to a destination without delay, by displaying various road information such as traffic snarl on a road map along a route of which the vehicle is guided to a destination, as disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 58-143370, for instance.
In the displaying systems of this sort, traffic snarl sections are usually displayed on the road map in color according to the traffic snarl conditions on the basis of road information transmitted from traffic radio beacon transmitters arranged along roads. Further, the displayed information such as traffic snarl sections can be updated whenever new road information is received from another radio beacon transmitted arranged at another position. The road information transmitted from the radio beacon transmitter includes position data and traffic snarl data, for instance. The position data indicates the position at which the radio beacon transmitter is arranged, and therefore provides information of the current position where the vehicle is now travelling. The traffic snarl data indicates the road sections where traffic is snarled due to rush hours, seasonal trips or journeys, accidents, construction, no passing, etc. Since traffic snarl varies momentarily, the most updated traffic snarl data must be provided for drivers. Further, since the above-mentioned traffic radio beacon transmitters are not necessarily arranged along all roads, there exists of course the case where the vehicle travels on the road along which no radio beacon transmitters are arranged.
In the prior-art road map displaying system for an automatic vehicle, however, there exists a problem in that preceding road information (e.g. traffic snarl) transmitted by a radio beacon transmitter is kept displayed on the road map, and therefore the driver is confused with the preceding traffic snarl information. The above-mentioned undesirable condition occurs when the vehicle first travels on a road along which radio beacon transmitters are arranged and then travels on another road along which no radio beacon transmitters are arranged. This is because the preceding road information is not updated by the succeeding radio beacon transmitter.